In a typical binary signaling system, a signal may only be transmitted at one of two signal levels (i.e., a logic zero signal level or a logic one signal level). Thus, in such a binary signaling system, a signal may only represent one bit of data at a time (i.e., a data bit at a logic zero signal level or a data bit at a logic one signal level). In contrast, in a multi-level signaling system, a signal may be transmitted at one of multiple signal levels. For example, in a 4-PAM (4-level Pulse Amplitude Modulation) signaling system, a signal may be transmitted at one of four signal levels (i.e., a logic zero signal level, a logic one signal level, a logic two signal level, or a logic three signal level). Thus, in such a 4-PAM signaling system, a signal may represent two bits of data at a time (i.e., two data bits (i.e., 00) represented by a logic zero signal level, two data bits (i.e., 01) represented by a logic one signal level, two data bits (i.e., 10) represented by a logic two signal level, or two data bits (i.e., 11) represented by a logic three signal level).
Referring to FIG. 1, the various signal waveforms for a single-ended 4-PAM signaling system are shown, along with reference levels (i.e., VrefH, VrefM, and VrefL), logic signal levels (i.e., in Gray code sequence 0, 1, 3, and 2), and logic signal level binary values (i.e., in Gray code sequence 00, 01, 11, and 10). The reference levels are used to determine most significant bits (MSBs) and least significant bits (LSBs) of signals in terms of the logic signal level binary values. That is, the MSB of a signal may be extracted by a simple comparison of the signal to the VrefM reference level. In contrast, the LSB of a signal must be extracted through a simultaneous comparison of the signal to both the VrefH and VrefL reference levels.
Referring to FIG. 2, there is shown a circuit 200 for extracting the LSB of a signal in a single-ended 4-PAM signaling system. As shown in FIG. 2, the circuit 200 comprises multiple transistors 202 and resistive elements 204. The circuit 200 receives a single-ended 4-PAM input signal (i.e., Vin), along with VrefH and VrefL reference level voltage signals and a bias voltage signal (i.e., VBias), and generates a pair of complementary output voltage signals (i.e., Vout and Voutb) indicating the state of the LSB in the single-ended 4-PAM input signal (i.e., Vin)
Single-ended multi-level signaling systems, such as the single-ended 4-PAM signaling system discussed above, are often implemented to alleviate signal attenuation problems which are frequently encountered in high-speed (e.g., above 5 Gb/s) serial link channels, which are often found in backplane environments. However, despite the benefits obtained through the use of single-ended multi-level signaling systems, further solutions may also be required to address such signal attenuation problems. One such solution is realized through the use of differential multi-level signaling systems due primarily to the benefits that differential signaling offers in the area of common-mode noise rejection.
Referring to FIG. 3, differential signal waveforms (i.e., Vin(+) and Vin(−)) for a differential 4-PAM signaling system are shown, along with reference levels (i.e., VrefH, VrefM, and VrefL), differential logic signal level binary values (i.e., in Gray code sequence 00, 01, 11, and 10), Vin(−) logic signal levels (i.e., in Gray code sequence 0, 1, 3, and 2), and Vin(−) MSB & LSB logic value ranges. Analogous to the case for the single-ended 4-PAM signaling system described above, the reference levels are used to determine MSBs and LSBs of differential signals in terms of the differential logic signal level binary values. That is, the MSB of a differential signal may be extracted by a simple differential comparison of the differential signal to the VrefM reference level. In contrast, the LSB of a differential signal must be extracted through a simultaneous comparison of the differential signal to both the VrefH and VrefL reference levels. This simultaneous comparison is preferably performed by a differential window comparator.
Referring to FIG. 4A, there is shown a circuit 400 for extracting the LSB and MSB of a differential signal in a differential 4-PAM signaling system. As shown in FIG. 4A, the circuit 400 comprises a preamplifier stage 402, a regenerative amplifier stage 404, and a Gray decoder stage 406. The circuit 400 receives a differential 4-PAM input signal (i.e., Vin(+) and Vin(−)), along with a Vref reference level voltage signal, and generates a pair of output voltage signals (i.e., VLSB and VMSB) indicating the states of the LSB and the MSB in the differential 4-PAM input signal (i.e., Vin(+) and Vin(−)).
Referring to FIG. 4B, there is shown a more detailed schematic diagram of the preamplifier stage 402 shown in FIG. 4A. Specifically, FIG. 4B shows that the preamplifier stage 402 comprises a plurality of transistors 408 and a plurality of resistive elements 410. The preamplifier stage 402 uses the single reference level voltage signal (i.e., Vref) to introduce offset into two different LSB-extracting transistor pairs.
Referring to FIG. 4C, the relationship between the reference level voltage signal (i.e., Vref) and the signal levels of the differential 4-PAM input signal (i.e., Vin(+) and Vin(−)) is shown.
As demonstrated above, in differential multi-PAM systems, the MSB of a differential signal may be extracted by a simple differential comparison of the differential signal to a single reference level (e.g., ground). In contrast, the LSB of a differential signal must be extracted through a simultaneous comparison of the differential signal to multiple reference levels. Further, in Gray code based systems, voltage differences in the differential signal are smaller in the region where the LSB=1, which occupies the center band of the differential signal. In contrast, in Gray code based systems, voltage differences in the differential signal are larger in the region where the LSB=0, which occupies the outer bands of the differential signal. Accordingly, a highly accurate differential window comparator and related circuitry is required for reliable LSB extraction operations.
In view of the foregoing, it would be desirable to provide a technique for receiving differential multi-PAM signals which operates in an efficient and cost effective manner.